Plug valves have received wide acceptance in many types of liquid flow control applications. Their wide use, particularly in the case of ball valves, is due in no small part to the ease and quickness of opening and closing. By making such valves of plastic materials, protection is afforded against damage by many corrosive fluids which makes such valves attractive for use in many process applications as well as in water lines.
A major problem associated with the manufacture of plastic ball valves of the prior art is that ball seats are formed as separate elements, usually of a special fluorinated hydrocarbon resin such Teflon. The use of separate seating rings adds to the cost and complexity of the valves. Moreover, with separate seating rings, leakage can develop through two paths: one between the rotary ball and the seating ring, and the other between the seating ring and the valve body or ring carrier.
Prior art efforts at providing a plug valve where the housing is molded with the ball in place are discussed in several patents, including U.S. Pat. No. 3,271,845 to Breher. While some forms of valves discussed in this patent and the valves shown in French Pat. No. 1,359,824 (1964) are disclosed as not having separate seat rings or seals, no such valve has found acceptability in the marketplace, apparently because of the lack of an effective sealing capability.
U.S. Pat. No. 3,807,692 to Usab et al discloses a commercially available valve having a housing molded with the ball in place, but in this valve separate seating rings of Teflon are positioned on opposite sides of the ball before the housing is molded. In U.S. Pat. No. 3,223,111 to Anderson, a seat or socket is shown that is molded in place around the plug to form an integral valve and seat unit. This integral valve and seat unit is thereafter placed in a metal housing and subjected to a compression which is stated to create a sufficient force to cause the plastic material of the seat unit to flow and establish, by compression, a seat with the spherical surface of the valve member.
In U.S. Pat. No. 3,712,584 to Wise, the housing is molded in place about a valve ball which is purposely made non-spherical so that the portion of the valve body housing which serves as a socket for the valve ball is formed with a special rigid, raised portion, which in one embodiment is shown to be rigid annular ridges surrounding the valve ball orifices. When the valve ball is rotated to a valve closed position, the rigid, raised portion on the housing frictionally engages and slides against the valve ball to be tightly forced against the valve body portion. To provide a seal, the valve ball must have a very small departure from true roundness which usually requires secondary operations on the ball before the housing is molded in place, thereby adding to the cost of the valve. Also, to the extent a tight fit between the housing and the valve ball is desired, there is required a significant turning force, particularly to break the ball away from its valve open position.